EP0056383B1 - Sensor - Google Patents

Abstract

The sensor for measuring the pressure of a medium comprises a wire (5) the resistance of which varies in accordance with the pressure. The wire (5) is placed within the medium in a coil or pleated form or the like. Preferably a portion (5a) of the wire is protected from the pressure. The protected portion (5a) of the wire and the portion (5b) exposed to the pressure are forming the branches of a bridge circuit. In order to measure the combustion pressure in combustion machines, circuits are located in the chamber (11) of a spark plug or disposed with the electric conductors (21) (44) of an incandescent plug. Combustion pressures may thereby be obtained by controlling and/or adjusting injection and/or ignition systems without further borings. In other embodiments of the sensor high pressure may also be measured, for example the pressure in an injection pump.

Description

State of the art

The invention is based on a sensor according to the type of independent claims 1 and 4.

Various arrangements are already known for measuring the pressure in a medium by detecting the action of force on a sensor arranged in the medium. Piezoceramic arrangements are suitable as sensors for this, but also arrangements with a wire that changes its resistance under pressure, as described, for example, in the textbook FX Eder, Modern Measurement Methods of Physics, Part I, VEB-Verlag der Wissenschaften 1968, page 259 . Manganin wire is particularly suitable as a pressure-sensitive wire.

In addition to considerable costs, the known sensors of the type mentioned have the disadvantage that their properties change significantly with temperature differences and they are not suitable for special tasks such as occur, for example, when the pressure in the combustion chamber of an internal combustion engine or in associated units is measured should.

Internal combustion engines are only apparently continuously operating machines. Although torque and speed are quasi-continuous output variables, they are generated by a rapid succession of individual processes.

In the course of optimizing the operation of internal combustion engines, in particular with a view to reducing fuel consumption, it is therefore necessary to grasp these relationships in detail in order to be able to influence the overall process in the desired manner by influencing the individual process.

The prerequisite for this is the determination of the state inside the internal combustion engine. Typical state variables are pressure and temperature, which indicate the overall state in the case of the ideal gas. The operation of engines with internal combustion is largely determined by chemical processes and material changes, so that this operation cannot be described globally with pressure and temperature, but the observation of pressure and temperature provides sufficient information about the changes in the state variables via the Engine cycle.

It is known in the combustion chamber of an internal combustion engine to record the temporal and spatial spread of the combustion process, the ignition timing, the detection of injection processes and the measurement of irregular combustion.

These irregular burns also include what is known as “knocking”, which occurs in internal combustion engines under certain working conditions. This means tone-frequency vibrations of the compressed fuel-air mixture, which are triggered by a shock wave. During these vibrations, the heat transfer to the piston and cylinder walls of the internal combustion engine is greatly increased. This results in damaging thermal overload of these surfaces, so that knocking must be avoided in principle. On the other hand, since efforts are made to utilize the available working area of the internal combustion engine as much as possible, it is necessary to provide means that indicate knocking early and safely in order to be able to implement a control of the internal combustion engine in which the internal combustion engine is always active is operated just below the knock limit.

From US-A-2 879 450 a sensor according to the type of independent claims 1 and 4 is already known, in which the electrical conductor, which changes its resistance value when the pressure in the combustion chamber of the internal combustion engine changes, is not directly pressure-sensitive. Rather, the electrical conductor changes its resistance value as a function of tensile stresses and is applied to a flexible, cylindrical wall which encloses an annular cavity around a spark plug in a pressure-tight manner, the cavity being connected to the breathing space of the spark plug via channels in the threaded region of the spark plug is located between the center electrode and the external thread and extends to the combustion chamber of the internal combustion engine. The pressure changes in the combustion chamber cause tensile stresses in the cylindrical wall, which change the resistance value of the electrical conductor applied to the wall. This known arrangement is particularly complex and expensive.

Advantages of the invention

The sensor according to the invention with the characterizing features of independent claim 1 has the advantage that a measurement in the combustion chamber of an internal combustion engine is possible with the simplest means, and a commercially available spark plug can be used as a carrier, which does not have to be provided with an outgoing bore .

The same advantage is achieved with the characterizing features of independent claim 4, wherein the electrical conductor of a glow plug, which is present anyway and is used for preheating, is used as a pressure-sensitive element.

A particularly good effect is also achieved in that a conductor used for preheating on a kerami serves as a conductor that changes its resistance under the influence of pressure the glow plug is used. Because this conductor track is surrounded on the one hand by a dense protective layer, there are extensive options for choosing the conductor material, so that a material can be selected in which the pressure and / or temperature influence is particularly reflected in the change in the resistance value.

drawing

• Figur 1 ein Ausführungsbeispiel eines Sensors ;
• Figur 2 eine Abwandlung einer Einzelheit der in Figur 1 dargestellten Sensoranordnung mit teilweiser Schirmung des Drahtes ;
• Figur 3 eine erfindungsgemäße Sensoranordnung in einer Zündkerze ;
• Figur 4 eine schematische Darstellung einer erfindungsgemäßen Sensoranordnung mit Auswerteschaltung ;
• Figur 5a und b einen Schnitt bzw. eine Ansicht einer keramischen Glühstiftkerze, wie sie bei einer Anordnung gemäß Figur 4 verwendet werden kann.

The invention is shown in the drawing and explained in more detail in the following description. Show it

• Figure 1 shows an embodiment of a sensor;
• Figure 2 shows a modification of a detail of the sensor arrangement shown in Figure 1 with partial shielding of the wire;
• FIG. 3 shows a sensor arrangement according to the invention in a spark plug;
• Figure 4 is a schematic representation of a sensor arrangement according to the invention with evaluation circuit;
• 5a and b show a section or a view of a ceramic glow plug as can be used in an arrangement according to FIG.

Description of the embodiments

In the embodiment of a sensor shown in FIG. 1, the wall of a container in which a pressure medium 2 is located is indicated by 1. In the wall 1, a cylinder screw 3 is arranged, which carries a support body 4 on the pressure chamber side, on which a wire 5 is wound, which is physically such that it changes its resistance depending on the pressure acting on it. Manganese wires are particularly suitable for this. The wire 5 is designed in the form of a coil, a meander or the like, so that there is a large length of wire in a small space and thus a large change in resistance occurs with changing pressure. The wire 5 is connected to a measuring arrangement not shown in FIG. In this measuring arrangement, the resistance value of the wire 5 is monitored in a manner known per se. The wall 1 can z. B. a cylinder wall or the housing wall of an injection pump, so that the combustion chamber pressure or the pressure in an injection pump can be measured with the arrangement according to FIG. However, it goes without saying that the arrangement described can also be used to measure other, preferably high, pressures. In the further embodiment of a sensor shown in FIG. 2, a bridge circuit is illustrated using the example of a coil arrangement. The wire 5 is in turn wound up in a coil-like manner on a carrier body 4, but a first part 5a of the wire is surrounded by a pressure-resistant shield 7 and the other part 5b of the wire is unshielded in the printing medium. The parts 5a, 5b of the wire are guided into the outside space via a three-wire feed line 6 '. In the outside area, parts 5a, 5b are connected, for example with two further resistors, to a Wheatstone full bridge circuit. Part 5a of the wire, which is preferably covered by a plastic or metal layer, serves as a reference resistor for compensating for temperature influences.

It goes without saying that the arrangements as coils or meanders are only mentioned by way of example and that an abundance of other geometrical designs of the wire, in which a large length of wire occurs in a small space, can also be used.

In the embodiment of a sensor according to the invention shown in FIG. 3, a spark plug is used which has a so-called breathing space 10 between the central electrode 8 and the external thread 9. This breathing space 10 is suitable for receiving a sensor arrangement according to the invention in such a way that, for example, a wire coil 5 is introduced on the inside of the external thread 9, which is preferably provided with a protective layer 11 and - if necessary - with an electrical shielding layer 12. In this case, the feed line 6 is led through the inside of the candle to the outside. With this arrangement it is possible to monitor the pressure in the combustion chamber of an internal combustion engine; since the pressure of the fuel gas is transferred to the coil 5. It is of course also possible to divide the coil 5 in the manner shown in FIG. 2 and described in detail above.

In FIG. 4, 21 denotes an electrical conductor, as is usually used in a glow plug for preheating in self-igniting internal combustion engines, in particular diesel engines. Such conductors 21 of glow plugs are known in various forms from the prior art. The conductor 21 of the glow plug is led to terminals 22, 23, through which it is supplied on the one hand with a current required for annealing from a current source not shown in the figure, but on the other hand, according to the invention, its resistance value is detected. According to the exemplary embodiment according to FIG. 4, this is done with the aid of a half-bridge circuit, which consists of the resistors 24 and 25 in cooperation with the electrical conductor 21. The resistors 24 and 25 are connected in series, one end point of this series connection and the center point being connected to switching means 26 and the other end point being connected to the terminal 22. The terminal 23 is also connected to the switching means 26. The switching means 26 initially comprise a known amplifier for small electrical signals, preferably an amplifier such as that for the evaluation of signals from strain gauges is used. The switching means 26 can also contain an evaluation circuit for knock signals, as described, for example, in DE-A-30 10 324, published on October 1, 1981.

It goes without saying that the conductor 21 changes its electrical properties not only under the influence of the pressure prevailing in the combustion chamber, but also under the influence of the temperature prevailing in the combustion chamber. Depending on the material used for the conductor 21, the arrangement shown in FIG. 4 can therefore be used in the same way for measuring the combustion chamber temperature or for analyzing the temperature profile with regard to the measurement of the combustion process.

A ceramic glow plug is shown in FIGS. 5a and b. The glow plug 30 has a tubular metal housing .31, the longitudinal bore of which is designated 32 and which has a screw thread 33, a hexagon key 34 and a sealing seat 35 arranged on the combustion chamber end section on the outside for installation in an engine (not shown). The longitudinal housing bore 32 is provided at its combustion chamber end section with a shoulder 36 on which a glow body 37 with an outwardly facing flange 38 rests; Between the glow plug flange 38 and the longitudinal bore shoulder 36 of the metal housing 31, a copper contact ring 39 is inserted, which also serves as a seal between the glow body 37 and the housing 31.

The incandescent body 37 has a ceramic tube 40 as a carrier, which is closed at its end section on the combustion chamber side by a bottom 41 and protrudes from the end section of the metal housing on the combustion chamber side. The ceramic tube 40 consists of electrically insulating ceramic material or glass ceramic, preferably of aluminum oxide, and has an outer diameter of approximately 5 mm at its end section protruding from the housing 31; the area of the bottom 41 has a wall thickness of 0.5 mm, but can also be between 0.3 and 0.8 mm thick, depending on the use of such a glow plug 30. In order to give the glow body 37 the lowest possible heat capacity, the wall thickness of the ceramic tube 40 preferably remains essentially the same up to the flange 38 and the ceramic tube interior 42 is left essentially empty. The bottom 41 is designed as a dome, but can also be of a different configuration.

The bottom 41 of the ceramic tube 40 is covered on its outer side with a thin porous, electrically insulating intermediate layer 43, which preferably consists of aluminum oxide, absorbs thermal expansion and prevents heat transfer from the heating element 44 to the ceramic tube 40 too quickly; the intermediate layer 43 can also be extended further on the ceramic tube 40 in the direction of the ceramic tube flange 38.

The heating element 44 is essentially limited to the area of the ceramic tube base 41, is layered and consists of a platinum-rodium alloy which is coated with ceramic material such as, for. B. alumina is added. Instead of the platinum-rodium alloy, other platinum metals, alloys of platinum metals or also other suitable electrically conductive substances (for example Ag perovskite) can be used for the heating element 44. As can be seen from FIG. 5b, the heating element 44 has a wavy configuration in the present example, which enables a high energy density. The heating element 44 takes up a smaller area on the ceramic tube base 41 than the intermediate layer 43. This heating element 44 is provided with an electrically insulating, dense protective layer 45 made of ceramic material, e.g. B. aluminum oxide, which protects it from abrasion, corrosion and short circuit.

The heating element 44 is connected at its two ends to a first conductor track 46 or a second conductor track 47, which consist of a mixture of platinum and aluminum oxide, but also of other platinum metals or alloys of platinum metals or also of other suitable electrically conductive substances ( e.g. Ag perovskite) and a ceramic material. The first conductor track 46 leads up to the end face 48 of the ceramic tube flange 38 and is connected there to a contact part 50 which terminates in a contact thread 51 on the side facing away from the combustion chamber, which is connected to the terminal 22. By contrast, the second conductor track 47 already ends behind the flange combustion chamber side 49 and is connected via the copper contact ring 39 to the metal housing 31, which in turn is connected to the terminal 23.

Through the half-bridge circuit consisting of resistors 24 and 25, the resistance value of the conductor track consisting of sections 44, 46, 47 is now detected and - as described in detail above - evaluated in switching means 26.

Since the ceramic tube 40 in the glow plug according to FIGS. 5a and b is transparent due to its small wall thickness, in a preferred embodiment of the invention an optical pickup in the form of a light guide rod 52 is also provided, which collects the light that passes through the ceramic tube 40 from the combustion chamber falls into the interior 42 thereof. The light detected via the light guide rod 52 is passed on in a manner known per se, converted into electrical signals and evaluated, as is indicated in FIG. 5 a by the connection of the light guide rod 52 to the switching means 26.

Claims (8)

1. Sensor for sensing the pressure in the combustion chamber of an internal combustion engine, using an electrical conductor which changes its resistance under the action of pressure and which is influenced by the pressure prevailing in the combustion chamber, characterised in that a directly pressure-sensitive conductor (5) is used as an electrical conductor which changes its resistance under the action of pressure and is located in a breathing space (10) located between the central electrode (8) and the outer thread (9) of a spark plug.

2. Sensor according to Claim 1, characterised in that the electrical conductor (5) is in the form of a wire.

3. Sensor according to Claim 2, characterised in that a part (5a) of the wire (5) is screened from pressure, and this part and the remaining part (5b) of the wire (5) each form one arm of a bridge arrangement.

4. Sensor for sensing the pressure in the compression chamber of an internal combustion engine having an electrical conductor which changes its resistance under the action of pressure and which is influenced by the pressure prevailing in the combustion chamber and having a circuit (2b) which senses the change in resistance, characterised in that the electrical conductor (21), serving for preheating, of a glow plug is used as the electrical conductor which changes its resistance under the action of pressure.

5. Sensor arrangement according to Claim 4, characterised in that the conductor (21) is in the form of a conductor track (44, 46, 47) of a ceramic sheathed-element glow plug (30).

6. Sensor arrangement according to Claim 5, characterised in that the conductor track (44, 46, 47) is essentially restricted to the area of the bottom (41) - on the combustion chamber side - of a ceramic tube (40) which has essentially an empty inner cavity (42) and forms the glow pin, the track being in the form of a layer and preferably being covered by an electrically insulating, dense protective layer (45).

7. Sensor arrangement according to Claim 6, characterised in that the conductor track (44) has an essentially wave-form configuration on the bottom (41) of the ceramic tube.

8. Sensor arrangement according to one of Claims 1 to 7, characterised in that the plug (30) moreover possesses an optical sensor (52).

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